Biological production of ethanol from waste gases with Clostridium ljungdahlii

ABSTRACT

A method and apparatus for converting waste gases from industrial processes such as oil refining, carbon black, coke, ammonia, and methanol production, into useful products is disclosed. The method includes introducing the waste gases into a bioreactor where they are fermented to various product, such as organic acids, alcohols H 2 , SCP, and salts of organic acids by anaerobic bacteria within the bioreactor. These valuable end products are then recovered, separated and purified.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under Cooperative Agreement Numbers DE-FC02-90CE40939 and DE-FC04-94AL98770 awarded by the Department of Energy. Hence, the U.S. Government may have certain license rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 08/258,446, filed Jun. 10, 1994 U.S. Pat. No. 5,593,886, issued Jan. 14, 1997, which is a Division of U.S. patent application Ser. No. 07/968,857, filed Oct. 30, 1992, now abandoned. This application is also a continuation-in-part of U.S. patent application Ser. No. 08/347,512, filed Nov. 30, 1994 now U.S. Pat. No. 5,807,722, which is a continuation of U.S. patent application Ser. No. 07/968,857, filed Oct. 30, 1992, now abandoned. This application is also a continuation-in-part of U.S. patent application Ser. No. 08/220,686, filed Mar. 31, 1994 now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to biological methods, processes, microorganisms, and apparatus for producing products, materials, intermediates, and the like such as organic acids, single cell protein ("SCP"), hydrogen, alcohols, and organic acid salts from the waste gas streams of certain industrial processes and more particularly concerns a process utilizing continuous gaseous substrate fermentation under anaerobic conditions to accomplish this conversion.

The conventional procedure for producing organic acids, alcohols, hydrogen and organic acid salts is chemical synthesis of petroleum-derived feedstocks. The rapidly escalating cost of petroleum has generated considerable interest in producing these valuable commodities by fermentative processes that utilize renewable or waste materials as the feedstock. Single cell protein is produced as a by-product of the fermentations to be used as an animal feed supplement.

There is also growing concern over the massive amounts of atmospheric pollutants and greenhouse gases produced by conventional industrial processes. The Environmental Protection Agency recently estimated that over six million metric tons of carbon monoxide and nearly four million metric tons of hydrogen were discharged annually by the industrial complex. A substantial portion of this waste carbon monoxide and hydrogen is the result of carbon black manufacture and coke productions roughly 2.6 million metric tone of CO and 0.5 million metric tons of H₂. Large amounts of carbon monoxide or hydrogen are also produced by the ammonia industry (125, 144 metric tons of CO in 1991), petroleum refining (8 metric tons per thousand barrels), steel mills (152 pounds per metric ton of steel produced), and sulfate pulping of wood (286 pounds per ton of pulp). In 1991, the adipic acid industry generated 40,773 metric tone of carbon monoxide that was burned for fuel value or flared. In many cases, these gases are discharged directly to the atmosphere, placing a heavy pollution burden on the environment.

Typically, the waste gases from the manufacture of industrial products are released at low pressures and temperatures. Current technology cannot utilize these dilute gases tinder such conditions. Adapting existing technology to separate and recover hydrogen or carbon monoxide from these waste streams would be expensive and impractical.

In light of the foregoing, there is a need for a cost effective and practical method, microorganism, and apparatus for utilizing the above-described waste gases and for producing products, materials, intermediates and the like such as organic acids, alcohols, hydrogen and organic acid salts by other than chemical synthesis of petroleum derived feedstocks.

SUMMARY OF THE INVENTION

In accordance with the present invention, products, materials, intermediate, and the like such as organic acids, alcohols, hydrogen, single cell protein and/or organic acid salts are produced from the waste carbon monoxide, hydrogen, and/or carbon dioxide of industrial processes, thereby reducing environmental pollution while at the same time saving energy and chemical feedstocks.

In accordance with an exemplary process of the present invention, the desired components of the dilute gas mixtures are introduced into a bioreactor containing one or more cultured strains of anaerobic bacteria that utilize the waste gas components by a direct pathway to produce a desired compound. The compound is recovered from the aqueous phase in a separate vessel or vessels, utilizing a suitable recovery process for the compound produced. Examples of recovery processes include extraction, distillation or combinations thereof, or other efficient recovery processes. The bacteria are removed from the aqueous phase and recycled to avoid toxicity and maintain high cell concentrations, thus maximizing reaction rates. Cell separation, if desired, is accomplished by centrifugation, membranous ultrafiltration, or other techniques.

The principal object of the present invention is the provision of a process and/or microorganism for the production of products, intermediates, materials, and the like such as organic acids, hydrogen, single cell protein, alcohols, and/or organic acid salts from carbon monoxide, hydrogen, and/or carbon dioxide.

Another object of the present invention is the provision of methods, microorganisms and apparatus for the production of items ouch as organic acids, alcohols, hydrogen, single cell protein and/or salts from the waste gas streams of industrial processes such as oil refining, carbon black, coke, ammonia, and methanol production.

A still further object of the present invention is the provision of a process for producing acetic acid and/or ethanol from a waste gas stream of identical composition to that found in the manufacture of carbon black.

Yet another and more particular object of the present invention is the provision of a method, microorganism and apparatus involving continuous gaseous substrate fermentation under anaerobic conditions to accomplish the conversion of waste gas streams of certain industrial processes into useful products such as organic acids including acetic acid, alcohols, hydrogen, single cell protein and organic acid salts.

Other objects and further scope of the applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings wherein like parts are designated by like reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a process for the production of acetic acid from waste gas.

FIG. 2 is a schematic diagram of a process for the production of CMA from waste gas.

FIG. 3 is a schematic diagram of a process for the production of ethanol from waste gas.

FIG. 4 is a schematic representation of a continuous fermentation system in accordance with an embodiment of the present invention.

FIG. 5 is a graphical illustration of cell concentration (OD) versus time.

FIG. 6 is a graphical representation of acetic acid (HAC) versus time.

DETAILED DESCRIPTION OF THE INVENTION

The term "waste gas" or "waste gas streams" as used herein means carbon monoxide and hydrogen mixed with other elements or compounds, including carbon dioxide, nitrogen and methane, in a gaseous state and which are typically released or exhausted to the atmosphere either directly or through combustion. Normally, release takes place tinder standard smokestack temperatures and pressures. Accordingly, the processes of the present invention are suitable for converting these atmospheric pollutants into useful products such as organic acids, alcohols and organic acid salts. These products include, but are not limited to acetic, propionic, and butyric acids; methanol, ethanol, propanol, and n-butanol; plus salts, such as calcium magnesium acetate (CMA) and potassium acetate (KA).

Anaerobic bacteria which are known to convert carbon monoxide and water or hydrogen and carbon dioxide into alcohols and acids and acid salts include Acetobacterium kivui, A. woodii, Clostridium aceticum, Butyribacterium methylotrophicum, C. acetobutylicum, C. formoaceticum, C. kluyveri, C. thermoaceticum, C. thermocellum, C. thermohydrosulfuricum, C. thermosaccharolyticum, Eubacterium limosum, C. ljungdahlii PETC and Peptostreptococcus productus. Anaerobic bacteria known to produce hydrogen from carbon monoxide and water include Rhodospirillum rubrum and Rhodopseudomonas gelatinosa.

More specifically, bacterial species such as Acetogenium kivui, Peptoetreptococcus productus, Acetobacterium woodii, Clostridium thermoaceticum and Eubacterium limosum produce acetate by the reaction:

    4CO+2H.sub.2 O→CH.sub.3 COOH+2CO.sub.2 dG=-39 kcal/reac.(1)

Many anaerobic bacteria are also known to produce acetic acid from H₂ and CO₂. These bacterial isolates include A. kivui, P. productus, and Acetobacterium sp., which utilize homoacetic fermentation by anaerobically oxidizing hydrogen and CO₂ according to the equation:

    4H.sub.2 +2CO.sub.2 →CH.sub.3 COOH+2H.sub.2 dG=-25 kJ/reac.(2)

Acetobacterium woodii and Acetoanaerobium noterae produce acetate from H₂ and CO₂ according to the above reaction, but in addition to acetate, A. noterae produces some propionate and butyrate. Another chemolithotrophic bacteria, Clostridium aceticum, produces acetate from CO₂ using a glycine decarboxylase pathway.

Some bacteria, like A kivui, P. productus, and A woodii, produce acetate from either CO and H₂ O or H₂ and CO₂. P. productus gives particularly fast rates of conversion and demonstrates high tolerance to CO; however, this organism shows a preference to follow Equation (1) over Equation (2).

In addition to these listed bacteria, two strains of an additional clostridia which produce acetic acid or ethanol from CO and H₂ O or H₂ and CO₂ have been isolated. One is a Clostridium ljungdahlii ERI2 rod-shaped, gram positive, non-thermophilic anaerobe which gives superior acetic acid yields and operates at a low pH, which greatly enhances the recovery of the product. C. ljungdahlii ERI2 carries out a vigorous acetogenic fermentation of glucose. It also infrequently forms spores and carries out a primarily acetogenic fermentation of hexose or H₂ :CO₂. It is motile with peritrichous flagellation. This new strain of C. ljungdahlii, referred to as ERI2, was isolated from a natural water source and was deposited on Dec. 8, 1992, with the American Type Culture Collection (ATCC), Rockville, Md., accession no. 55380.

In preparing the products of the present invention, "mixed strains" of the bacteria enumerated hereinabove may be utilized. By mixed strains, it is meant a mixed culture of two or more anaerobic bacteria. This mixed strain, when utilized in the process described herein, produces organic acids (such as acetic acid and the like) or salts thereof, alcohols, hydrogen, SCP, etc.

In the development of the present invention, new strains of anaerobic bacteria have been isolated which enact this conversion with high efficiency. In addition, modifications to the fermentation conditions can result in the production of ethanol instead of acetic acid in some strains. Depending on the specific microorganism(s) utilized, variables which must be considered in forming products from waste gases include nutrient constituents and concentrations, medium, pressure, temperature, gas flow rate, liquid flow rate, reaction pH, agitation rate (if utilizing a Continuously Stirred Tank Reactor), inoculum level, maximum substrate (introduced gas) concentrations to avoid inhibition, and maximum product concentrations to avoid inhibition.

In accordance with an exemplary embodiment of the present invention and as shown in FIG. 1 of the drawings, a first step in the conversion process is the preparation of nutrient media (10) for the anaerobic bacteria. The content of the nutrient media will vary based on the type of anaerobe utilized and the desired product. The nutrients are constantly fed to a bioreactor or fermenter (12), consisting of one or more vessels and/or towers of a type which includes the Continuously Stirred (CSTR), Immobilized Cell (ICR), Trickle Bed (TBR), Bubble Column, Gas Lift Fermenters, or other suitable fermentation reactor. Within bioreactor (12) resides the culture, either single or mixed species, of anaerobic bacteria utilized in the gas conversion process. For the CSTRS, TBRs, Bubble Columns and Gas Lift Fermenters, these bacteria live dispersed throughout the liquid phase of the reactor, but for ICRs, the bacteria adhere to an internal packing medium. This packing medium must provide maximal surface area, high mass transfer rate, low pressure drop, even gas and liquid distribution, and must minimize plugging, fouling, nesting and wall channeling. Examples of such medium materials are ceramic Berl saddles, Raschig rings or other high performance packings.

The waste gases (14) are continuously introduced into the bioreactor (12). The gas is retained in the bioreactor (12) for the period of time which maximizes efficiency of the process. Exhaust gases (16), containing inerts and unreacted substrate gases, are then released, The liquid effluent (18) is passed to a centrifuge, hollow fiber membrane, or other filtration device (20) to separate out microorganisms that are entrained. These microorganisms (22) are returned to the bioreactor (12) to maintain a high cell concentration which yields a faster reaction rate (cell recycle).

A next step in the process is separation of the desired biologically produced product(s) from the permeate or centrifugate (24). In the embodiment depicted in FIG. 1, the permeate or centrifugate (24) is passed to an extraction chamber (26) where it is contacted with a solvent (28). The solvent (28) should have a high distribution coefficient for the desired end product, a high recovery factor, low toxicity to humans, low toxicity to the bacteria, immiscibility with water, appropriately high boiling point, and form no emulsion with the bioreactor constituents. The distribution of solute between solvent and aqueous phase will determine the thermodynamic feasibility and the amount of solvent required to remove the end product. Typical solvents include secondary and tertiary amines in a suitable solvent, tributyl phosphate, ethyl acetate, tri-octyl phosphine oxide and related compounds in a suitable co-solvent, long chain alcohols, hexane, cyclohexane, chloroform, and tetrachloroethylene.

The nutrients and materials in the aqueous phase (30) pass back to the bioreactor (12) and the solvent/acid/water solution (32) passes to a distillation column (34), where it is heated to a sufficient temperature to separate the solvent (28) from the acid and water (36). The solvent (28) passes from the distillation column (34) through a cooling chamber (38) to lower the temperature to the optimum temperature for extraction, then back to the extraction chamber (26) for reuse. The acid and water solution (36) passes to a final distillation column (40) where the desired end product (42) is separated from the water and removed. The water (44) is recirculated for nutrient preparation.

FIG. 2 shows a process for the production of the road deicer calcium magnesium acetate (CMA) (46), from waste gas (48). The process is identical to the acetic acid process of FIG. 1 through solvent extraction. That is, identical organisms, nutrients and process conditions are used in continuous fermentation, including the reaction vessels. Similarly, cell recycle by hollow fiber membrane, centrifugation or other filtration device is identically employed in the process. Finally, the extraction of acetic acid in an extraction chamber, followed by recycle of the acid-free medium, is employed.

After extraction, the process for producing CMA differs greatly from the acetic acid production process of FIG. 1. In the CMA process the solvent (50) containing acetic acid and a small amount of water is sent to a reaction vessel (52) for CMA production. The water content of the solvent stream is dependent upon the solvent used for acetic acid extraction. Again, solvents such as secondary and tertiary amines in a suitable co-solvent, tributyl phosphate, ethyl acetate, tri-octyl phosphine oxide and related compounds in a suitable co-solvent, long chain alcohols, hexane, cyclohexane, chloroform and tetrachloroethylene may be employed with varying success. The reaction vessel (52) for CMA is most suitably a Continuous Stirred Tank Reactor (CSTR), although other reactor systems may be employed. A mixture (54) of dolomitic lime and magnesium oxide in water is added to the solvent containing acetic acid and water. Reaction occurs to produce CMA in aqueous solution at or below the saturation level.

The CMA, water and solvent (56) are then sent to a settling device (58) to separate the aqueous and solvent phases. The solvent phase (60) is returned to the extraction chamber for recycle. The CMA/water (62) is sent to drying/pelletizing (64) to produce a pelletized CMA product.

Potassium acetate (KA) can be produced as an alternative product by substituting caustic potash (or potassium oxide) for the dolomitic lime. Since KA is produced as a 50 percent aqueous solution, drying and pelletizing are not required.

FIG. 3 shows a process for the production of ethanol from waste gas. As in FIG. 1, waste gas (66) and nutrients (68) are fed into a reactor (70) containing a culture of microorganisms. The reactor may be any of the types described above in the narrative of FIG. 1. The organism used in the ethanol production process must be capable of producing ethanol in place of acetic acid/acetate. In general, a low fermentation pH of 4.0-5.5 is required, coupled with a nutrient limitation. The bacteria listed hereinabove which are capable of operating at these reduced pH levels can be used in this process of ethanol production.

Waste gas is fed into the reactor containing the culture of organisms capable of ethanol production along with the required nutrients. Ethanol is produced as the product in a similar fashion as in FIG. 1. Cell recycle (72) may be used to enhance the cell concentration in the reactor, but this operation is not required to make the process work. The permeate (74) from the cell recycle apparatus containing dilute ethanol in medium is sent to distillation (76), where the water (78) and ethanol (80) are separated. 95 percent ethanol exits the top of the distillation column and water (spent medium) exits the bottom of the column. The spent medium is sent back to the reactor as water recycle. The 95 percent ethanol is sent to a molecular sieve system (82) to produce anhydrous ethanol (84).

Thus in accordance with the present invention it is now possible to produce valuable organic acids, alcohols, or organic acid salts by a gaseous substrate fermentation, not only reducing consumption of valuable chemical feedstocks, but also removing hazardous atmospheric pollutants from the waste gas streams of many industries. Previous processes to derive these chemicals biologically were based on fermentation of sugars.

In the processes described hereinabove, it is preferred that the process is conducted at higher than 1 atmosphere. Preferably, it is preferred that it be conducted at pressures up to 30 atmospheres and more preferably up to 20 atmospheres and most preferably up to 15 atmospheres.

The following specific examples are submitted to illustrate but not to limit the present invention. Unless otherwise indicated, all parts and percentages in the specification and claims are based upon volume.

EXAMPLE 1

Production of Acetic Acid from Carbon Black Waste Gases

This example is directed to a process utilized to convert waste gas of a composition which matches that of the furnace exhaust of carbon black manufacture to acetic acid. The waste gas has a composition of about 13 percent carbon monoxide, 14 percent hydrogen, and 5 percent carbon dioxide, with the remaining 68 percent largely nitrogen with traces of oxygen and sulfur compounds. The waste gases are produced as the result of partial oxidation of gas or oil with insufficient air to form amorphous carbon, with about 1.2 pounds of carbon monoxide produced per pound of elemental carbon. These waste gases form a serious atmospheric contamination problem and also represent a valuable chemical feedstock resource not presently being recovered.

In the development of the present process, two distinct routes to produce acetic acid from carbon black waste gases were studied. The direct route converts CO and H₂ O or H₂ and CO₂ directly into acetic acid according to Equations (1) and (2), respectively. An indirect route involves the conversion of CO and H₂ O into H₂ and CO₂ by the water gas shift reaction, followed by production of acetic acid from H₂ and CO₂. This indirect route was found to be a less efficient utilization of the technology.

The acetogens tested are summarized in Table 1. Among these bacteria that produce acetic acid directly from CO, A. kivui and the newly isolated strain, C. ljungdahlii ERI2, show far superior rates for both CO and H₂ utilization. Further experimentation proceeded using these two anaerobic bacteria.

There are obvious advantages to the bacteria utilizing carbon monoxide and hydrogen simultaneously. This would afford the most efficient use of the waste gases and remove the greatest amount of atmospheric pollutants.

Bench Scale Operation of the Described Process to Produce Acetic Acid

As shown in FIG. 4 of the drawings and in accordance with one embodiment of the present invention, a bench scale continuous conversion system is shown to include a BioFlo IIC fermentor (150) from New Brunswick Scientific Co., Inc., Edison, N.J. The fermentor (150) is equipped with an agitation motor, pH controller, foam controller, thermostat, dissolved oxygen probe, nutrient pump, and 2.5 L culture vessel. The working volume is variable (1.5-2.0 L). Other variable operational parameters include medium feeding rate (Dilution rate), gas flow rate (Gas retention time), agitation (rpm). The vented or exhaust gases exit the fermentor (150) through a condenser fixed to a vented hood via a water trap and a sampling port.

The culture broth (152) is recycled through a cross-flow hollow fiber module (154) by a peristaltic pump (from Cole Parmer). The recycling rate is about 80-100 mL/min. The hollow fiber module (154) has the following characteristics; the surface area is 0.35 ft², the pore size is 0.2 um and the lumen diameter is 1 mm. The permeate (156) is pumped to a storage tank (150) (Feed storage). The culture cells are returned to the fermenter along line (155).

A counter-current acetic acid extraction system, including two stage mixer and settler components includes first and second mixers (160) and (162) and first and second settling tanks (164) and (166). The permeate (168) from storage (158) is pumped to mixer (160) through a flow controller (170). The solvent (172) is pumped to mixer (162) from solvent storage (174) through a flow controller (176). Both mixer (160) and mixer (162) are equipped with a stirring mechanism to achieve good mixing of aqueous phase and solvent phase. The mixture of both phases from the mixers (160) and (162) is led to settlers (164) and (166), respectively. The phase separation is accomplished in the settlers. The aqueous phase (178) from settler (164) is pumped to mixer (162), the solvent phase (180) from settler (164) is pumped to a separator (182), the aqueous phase (184) form settler (166) is pumped to raffinate storage (186), and the solvent phase (188) from settler (166) is pumped to mixer (160). The raffinate is recycled to the CSTR 50 along a line (190). This recycle line (190) is partially bled at (192) to remove inhibiting factors.

The solvent (180) loaded with acetic acid is pumped to a distillation flask (194) through a preheater (196). The distillation flask (194) is equipped with two thermocouples (196) and (198) to monitor and control temperature in the liquid phase and gas phase. The heating temperature for distillation is set to achieve maximum vaporization of the acetic acid. The acetic acid vapors are condensed in a condenser (100) and collected in a flask (102). The stripped solvent (104) is pumped through a cooling coil (106) to solvent storage (174).

A bench scale operation of the described process an diagrammed in FIG. 4 was fabricated in the laboratory to determine quantitative yields under optimized conditions. The nutrient mixture fed to the culture was as follows:

1. 80.0 ml of a salt, composed of

KH₂ PO₄ 3.00 g/L

K₂ HPO₄ 3.00 g/L

(NH₄)₂ SO₄ 6.00 g/L

NaCl 6.00 g/L

MgSO₄.2H₂ O 1.25 g/L

2. 1.0 g of yeast extract

3. 1.0 g of trypticase

4. 3.0 ml of PFN (Pfenning) trace metal solution

FeCl₂ *4H₂ O 1500 mg

ZnSO₄ *7H₂ O 100 mg

MnCl₂ *4H₂ O 30 mg

H₃ BO₃ 300 mg

CoCl₃ *6H₂ O 200 mg

CuCl₂ *H₂ O 10 mg

NiCl₂ *6H₂ O 20 mg

NaMoO₃ *2H₂ O 30 mg

Na₂ SeO₃ 10 mg

Distilled water 1000 ml

5. 10.0 ml of B vitamins

Pyridoxal HCl 10 mg

Riboflavin 50 mg

Thiamine HCl 50 mg

Nicotinic acid 5n mg

Ca-D-Pantotheinate 50 mg

Lipoic Acid 60 mg

P-aminobenzoic acid 50 mg

Folic acid 20 mg

Biotin 20 mg

Cyanocobalamin 50 mg

Distilled water 1000 ml

6. 0.5 g of Cysteine HCl

7. 0.06 g of CaCl₂.2H₂ O

8. 2.0 g of NaHCO₃

9. 1.0 ml of Resazurin (0.01%)

10. 920.0 ml of distilled water

For use with A kivui, the nutrient solution was pH adjusted to 6.6, whereas for the new strain, C. ljungdahlii ERI2, the pH was adjusted to 4.9. The ability to operate at a lower pH is a great advantage in acetic acid recovery. The solution was then sparged for 20 minutes with a 20% CO₂ and 80% N₂ atmosphere, then transferred anaerobically and autoclaved for 15 minutes.

Numerous experiments were carried out with both Continuous Stirred Reactors (CSTR) and immobilized Cell Reactors (ICR). The results obtained are exemplified in the following data.

CSTR Experiments Utilizing the bacterial Strains A. kivui and C. ljungdahlii ERI2

The bench scale system operating with the CSTR and the anaerobic bacteria, C. ljungdahlii ERI2 and A. kivui, consisted of a New Brunswick Scientific Bioflo IIc fermenter, a hollow fiber membrane unit for cell recycle, and extraction and distillation columns. Nutrient mixture was fed into the bioreactor at a rate of 3.2 cubic centimeters per minute. Capacity of the reactor was 2.5 liters, within which a constant fluid level of 1.5 liters was maintained. The fluid was agitated at variable rates of up to 1000 revolutions per minute with gas introduced at a rate of approximately 500 cubic centimeters per minute. Optimal gas retention times were in the range of three minutes. The gas teed varied with its uptake by the bacteria, which was in turn a function of the cell density.

The liquid from the bioreactor was passed to the hollow fiber membrane at a rate of 55 to 70 milliliters per minute. From the hollow fiber membrane, permeate was gathered at a rate of 1.5 milliliters per minute. Analysis of this permeate indicates the acetic acid/acetate concentration at this stage to range in excess of 20 grams per liter. Operating at a pH of 4.9, 42 percent of this product was in the acid form using C. ljungdahlii ERI2. For A. kivui, the acid yield was only 1.4 percent. Results of various runs for the two bacteria, including conversion rates and product yields are summarized in Tables 2 and 3.

ICR Experiments Utilizing the Bacterial Strain C. ljungdahlii ERI2

An immobilized Cell Reactor (ICR), consisting of a 2 inch outside diameter by 24 inch tall glass tube packed with fabric to support the cells and Enkamat 7020 as an immobilizing medium was also tested in the acetic acid production process. With C. ljungdahlii ERI2 as the acetogenic anaerobe, 100 percent of the carbon monoxide and 79 percent of the hydrogen were converted at a gas retention time of 20 minutes. Acetic acid concentrations in the removed liquid were approximately 6.0 grams per liter. Results of the ICR studies are summarized in Table 4.

The ICR has a certain attractiveness on an industrial scale in that the energy costs to operate the reactor are reduced significantly. The proper selection of packing materials, solution phases, and pressures may yield production approaching that of the CSTR.

Acetic Acid Recovery

Various solvents were tested for recovering acetic acid from the permeate, the results are summarized in Table 5. Tributyl phosphate was identified as having both a high distribution coefficient and a high boiling point. The solvent and permeate from the cell separator were commingled in a two stage extraction process, Alternatively, an extraction column could be used. Permeate was introduced into a 3 liter flask where it was mixed with incoming solvent. A ratio of 1 part solvent to 1 part permeate worked well and gave high recovery rates. The combined fluids were passed from the mixer to a 4 liter settling chamber where the solvent/acetic acid mixture separate as a lower density phase from the water and nutrients. Retention times of approximately 15 minutes were used in the settling tanks. The lower density phase was extracted and fed to a distillation flask. The raffinate was passed from the first settler to a second mixer where it was contacted again with solvent, then removed to a second settling chamber. This allowed for more complete extraction of the acetic acid; acid recovery increased from 82 percent to greater than 96 percent using tributyl phosphate. The solvent/acetic acid mixture from this settler was returned to the first mixer, while the raffinate of water and organics was passed back to the bioreactor.

The distillation unit was a 5 liter flask with a boiling mantle. A common distillation column, with reflux, could be used for complete acid recovery. Because of the high boiling point of tributyl phosphate, nearly complete recovery is accomplished in one step. The solvent/acetic acid mixture was heated to 120 degrees C, with the acetic acid collected overhead in a condensing coil. In this single stage system, distillation efficiencies of 10 percent were achieved.

Solvent mixtures were also tried and distribution coefficients of mixed solvents are summarized in Table 6.

EXAMPLE 2

Production of Acetic Acid from Carbon Black Waste Gases at Higher Pressures

Mass transport in the cellular reactions can be further enhanced by operating the system at increased pressures. Simple batch experiments were carried out to test the dynamics of this system. It was found that reaction rates increased in linear proportion to the pressure, with a corresponding reduction in effective retention time. Another advantage to operating at increased pressure is that reactor volume can also be reduced in linear fashion, i.e. operation at 10 atmospheres pressure requires a reactor with one tenth the volume of a reactor operating at 1 atmosphere. FIGS. 5 and 6 show the increase in cell density and acetic acid concentration, respectively, with the increased pressure. This acetic acid concentration far exceeds typical batch concentrations for a batch reactor at atmospheric pressure,

EXAMPLE 3

Production of Acetic Acid from Carbon Black Waste Gases with Surfactants

Mass transport is also increased by the use of surfactants. Table 7 presents the results of carbon monoxide uptake tests performed on C. ljungdahlii ERI2 in the pressure of various commercial surfactants. In each case, the control value of 100 (percent) represents CO uptake in batch fermentation, and the sample value, the percentage of the control in batch fermentation in the presence of the surfactant.

                  TABLE 1                                                          ______________________________________                                         ACETOGENIC BACTERIA TESTED FOR CO, H.sub.2, AND CO.sub.2                        CONVERSION                                                                        Bacterial Route                                                                             Simultaneous Consumption of CO and H.sub.2                    ______________________________________                                         Direct Route                                                                     P. productus No                                                                E. limosum No                                                                  A. noterae No                                                                  C. aceticum No                                                                 C. thermoaceticum No                                                           S. sphaeroides No                                                              A. woodii Yes                                                                  A. kivui Yes                                                                   C. ljungdahlii ERI2 Yes                                                        Indirect Route                                                                 R. gelatinosa No                                                               R. rubrum No                                                                 ______________________________________                                    

                                      TABLE 2                                      __________________________________________________________________________     Summary of ERI2 Experiments in the CSTR with Cell Recycle                      Gas Liquid  Percent                                                                              Dry Cell                                                                            Product                                                   Reten. Dilution Agitat. Gas Weight Concentration Specific                    Time                                                                               Rate                                                                               Rate                                                                               Conversion                                                                           Conc.                                                                               HAC                                                                               ETOH                                                                               Productivities                                   (min)                                                                              (hr.sup.-1)                                                                        (rpm)                                                                              CO H.sub.2                                                                           (g/L)                                                                               (g/L)                                                                             (g/L)                                                                              (g/L hr)                                                                           (g/g hr)                                     __________________________________________________________________________     9.30                                                                               0.056                                                                              750 80.75                                                                             74.5                                                                              2.3  9.7                                                                               0.07                                                                               0.43                                                                               0.18                                           9.28 0.055 750 82.1 72.0 3.32 9.56 0.094 0.52 0.16                             6.14 0.061 750 73.6 46.5 4.11 12.78 0.125 0.78 0.19                            6.4 0.08 750 74.8 49.6 5.02 12.98 0.125 1.05 0.19                              6.74 0.087 750 68.5 37.2 4.79 12.38 0.125 1.08 0.23                            4.91 0.10 750 68.8 50.2 4.53 10.73 0.05 1.08 0.24                              4.05 0.102 750 65.5 58.1 5.27 11.49 0.076 1.17 0.22                            3.98 0.103 900 74.3 67.9 6.17 12.73 0.1 1.31 0.21                              2.89 0.117 900 66.1 33.9 5.91 11.69 0.04 1.38 0.23                             3.28 0.105 1000  74.6 51.3 7.30 12.83 0.13 1.35 0.18                           3.22 0.125 1000  73.1 54.0 10.25 13.57 0.06 1.71 0.17                          2.63 0.13 1000  68.9 44.0 11.0 14.63 0.12 1.90 0.17                            2.3 0.134 1000  66.0 38.7 11.1 20.59 0.113 2.77 0.25                           2.7 0.11 1000  72.7 67.7 8.37 25.62 0.27 2.88 0.34                             2.4 0.11 1000  68.6 63.3 9.83 25.62 0.36 2.95 0.30                             2.53 0.122 1000  72.1 67.4 9.82 25.62 0.72 3.12 0.32                           3.0 0.13 1000  76.6 73.3 12.4 22.33 0.52 2.90 0.23                           __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     Summary of A. kivui Experiments in the CSTR with Cell Recycle                  Gas Liquid  Percent                                                                              Dry Cell                                                       Reten. Dilution Agitat. Gas Weight Product Specific                            Time Rate Rate Conversion Conc. Conc. Productivities                         (min)                                                                              (hr.sup.-1)                                                                        (rpm)                                                                              CO H.sub.2                                                                           (g/L)                                                                               (g/L)                                                                              (g/L hr)                                                                            (g/g hr)                                       __________________________________________________________________________     5.0 0.058                                                                              750 67.8                                                                              44.2                                                                              4.00 16.15                                                                              0.96 0.24                                             4.4 0.958 750 65.7 38.5 4.8 16.63 0.94 0.19                                    4.3 0.058 900 71.3 40.7 4.5 17.03 0.99 0.21                                    3.72 0.058 900 69.0 37.3 5.14 19.16 1.13 0.22                                  3.72 0.076 900 70.3 41.1 5.28 16.17 1.21 0.23                                  3.2 0.076 900 66.4 41.4 5.71 16.85 1.23 0.23                                   2.8 0.076 900 61.5 29.1 5.00 16.16 1.22 0.23                                   2.8 0.076 1000  69.5 36.3 5.8 18.58 1.62 0.29                                  2.8 0.11 1000  70.2 41.6 5.9 18.4 1.84 0.36                                    2.2 0.11 1000  64.0 25.0 7.2 16.5 2.1 0.3                                    __________________________________________________________________________

                  TABLE 4                                                          ______________________________________                                         Fabric ICR Performance with ERI2                                                 Liquid  Gas                           Product                                  Dilution Retention H.sub.2 CO Cell Concentration                             Rate  Time     Conversion                                                                               Conver-                                                                               Conc. HAC  ETOH                                  (hr) (min) (%) sion (%) (g/L) (g/L) (g/L)                                    ______________________________________                                         0.23  4.83     38.62     54.66  .125  3.221                                                                               .778                                   7.41 49.15 70.87 .120 2.690 .620                                               11.66 51.31 80.61 .067                                                         13.61 56.87 83.93 .064 2.099 .201                                             0.17 6.39 48.15 73.27 .161 3.382 1.365                                          11.21 68.96 92.62 .143 3.189 .495                                              55.44 83.13 96.27 .112 .813 .058                                              0.12 6.26 43.89 70.76 .094 3.864 1.689                                         0.09 7.87 42.40 79.72 .095 4.423 2.733                                          19.82 59.63 92.92 .102                                                        0.03 22.14 55.01 94.21 .071 4.878 2.631                                         29.00 78.60 100 .018 5.604 2.743                                               60.48 83.33 100                                                             ______________________________________                                    

                  TABLE 5                                                          ______________________________________                                         Acetic Acid Distribution Coefficient Study                                                        Equilibrium Aqueous                                                                          Acetic Acid                                      Acetic Acid Distribution                                                      Solvent Concentration g/L Coefficients                                       ______________________________________                                         Hexane         6.559         0.0                                                 Decane 5.968 0.08                                                              Chloroform 5.128 0.09                                                          Kerosene 4.648 0.11                                                            Hexadecane 5.866 1.13                                                          Dodecane 4.654 0.13                                                            Dodecyl acetate 5.787 0.15                                                     Dibutyl phosphate 4.615 0.18                                                   Oleyl alcohol 5.114 0.28                                                       Trioctylamine 3.785 0.31                                                       Undecyl alcohol 4.528 0.40                                                     Ethyl acetate 4.550 0.41                                                       Ethyl butyrate 4.665 0.42                                                      Dexyl alcohol 3.890 0.42                                                       Octanol 4.358 0.45                                                             Nonyl alcohol 3.470 0.55                                                       2-ethyl-1-hexanol 3.308 0.77                                                   3-methylcyclohexanol 2.110 1.26                                                Cyclohexanone 2.702 1.66                                                       Tributyl Phosphate 1.657 2.38                                                ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                         Distribution Coefficients of Mixed Solvents                                                             Distribution                                                                             Percent                                       Solvent Mix Coefficients Increase                                            ______________________________________                                         Oleyl Alcohol (10 cc)                                                                               0.17                                                        Oleyl Alcohol (10 cc) + Cyc (1 cc) 0.31  72                                    Oleyl Alcohol (10 cc) + TBF (1 cc) 0.29  61                                    Oleyl Alcohol (10 cc) + Cyc (2 cc) 0.45 150                                    Oleyl Alcohol (10 cc) + TBF (2 cc) 0.42 133                                    Oleyl Alcohol (10 cc) + Cyc (3 cc) 0.36 100                                    Oleyl Alcohol (10 cc) + TBF (3 cc) 0.42 133                                    Oleyl Alcohol (10 cc) + Cyc (4 cc) 0.35  94                                    Oleyl Alcohol (10 cc) + TBF (4 cc) 0.40 122                                    Oleyl Alcohol (10 cc) + Cyc (6 cc) 0.52 188                                    Oleyl Alcohol (10 cc) + TBF (6 cc) 0.65 261                                    Oleyl Alcohol (10 cc) + Cyc (7 cc) 0.69 283                                    Oleyl Alcohol (10 cc) + TBF (7 cc) 0.74 311                                  ______________________________________                                    

                  TABLE 7                                                          ______________________________________                                         CO CONSUMPTION BY ER12 IN THE                                                    PRESENCE OF SURFACTANTS                                                                                    With                                               Control* Surfactant                                                          ______________________________________                                         DNAP (0.1%, v/v)     100      0                                                  Nondiet P-40 (0.1%, v/v) 100 0                                                 Tergitol NP-10 (0.1%, v/v) 100 0                                               Tergitol Min Foam 1X (0.1%, v/v) 100 0                                         Tergitol TMN-10 (0.1%, v/v) 100 0                                              Triton X-15 (0.1%, v/v) 100 0                                                  Triton X-100 (0.1%, v/v) 100 0                                                 Triton X-114 (0.1%, v/v) 100 0                                                 Triton N-101 (0.1%, v/v) 100 5.83                                              Triton X-405 (0.1%, v/v) 100 7.82                                              Tergitol 8 (0.1%, v/v) 100 12.15                                               Triton N-42 (0.1%, v/v) 100 42.90                                              Witconol NS-500K (0.1%, w/v) 100 79.08                                         Tween 85 (0.1%, v/v) 100 82.16                                                 Witconol H-33 (0.1%, v/v) 100 90.12                                            Witconol 6903 (0.1%, v/v) 100 92.39                                            Tween 80 (0.1%, v/v) 100 97.15                                                 Arlacel 83 (0.1%, v/v) 100 97.43                                               Span 80 (0.1%, v/v) 100 99.12                                                  Tyloxapol (0.1%, v/v) 100 104.86                                               Witconol 5906 (0.1%, v/v) 100 108.42                                           Span 85 (0.1%, v/v) 100 124.85                                                 W-1 (0.001%, w/v) First time 100 105.89                                        Second time regas 100 0                                                        Brij 96 (0.004%, w/v) First time 100 107.98                                    Second time regas 100 0                                                      ______________________________________                                    

EXAMPLE 4

Production of CMA from Carbon Black Waste Gas

Carbon black waste gas containing about 14 percent Co, 17 percent H₂ and 4 percent CO₂, as the major components in N₂ is sparged into a 160 L continuous stirred tank reactor, maintained at 6 atm 37° C., and containing Clostiridium ljungdahlii isolate ERI2 ATCC deposit 55380. The waste gases are produced as the result of partial oxidation of hydrocarbons with insufficient air to form amorphous carbon, with about 1.2 pounds of carbon monoxide produced per pound of elemental carbon. These waste gases form a serious atmospheric contamination problem and also represent a valuable chemical feedstock resource not presently being recovered. The gas retention time (defined as the ratio of the reactor volume to the gas flow rate at standard conditions) is maintained at 0.52 min. An aqueous liquid medium containing water, base salts, B-vitamins, a nitrogen source and a sulfide source is fed to the reactor at a liquid dilution rate (defined as the ratio of the liquid flow rate to the reactor volume) of 1.05 hr⁻¹. The agitation rate in this reactor is 322 rpm, the temperature is 37° C. and the operating pH in 5.03. Under these conditions, the conversion of CO was 83 percent and the conversion of H₂ was 54 percent. A hollow fiber membrane cell recycle unit is used to maintain a cell concentration of 10.5 g/L inside the reactor. The dilute acetic acid/acetate product stream from the reactor containing 13.2 g/L acetic acid/acetate is sent to a three stage countercurrent extraction device, where it is extracted with solvent. The solvent to feed ratio is 1 to 4. The acetic acid in the acetic acid/acetate product stream is 3.1 g/L. The acetic acid concentration in the solvent leaving the extractor is 16.7 g/L. Water (medium) from extraction is sent back to the fermenter as recycle.

Dolomitic lime/MgO is added to the acetic acid directly in the solvent phase to form CMA. After reaction the saturated CMA solution is sent to drying and pelletizing 1.15 lb of CMA containing a Ca²⁺ /Mg²⁺ molar ratio of 3/7 is formed per pound of acetic acid.

EXAMPLE 5

Production of Acetic Acid from Carbon Black Waste Gas

Carbon black waste gas containing about 14 percent CO, 17 percent H₂ and 4 percent CO₂ in N₂ is sparged into a 144 L trickle bed reactor operating at 1.58 atm, 37° C. and containing Clostridium ljungdahlii isolate ERI2 ATCC deposit 55380. A trickle bed reactor is a column packed with a commercial packing such as Raschig rings or Berl saddles in which liquid and gas are contacted with each other due to flow through the column. In the present example, the liquid and gas both enter the column from the top in a cocurrent fashion, although countercurrent flow (gas entering the bottom, liquid entering the top) is possible. The gas retention time is maintained at 0.46 win and the liquid medium dilution rate is 0.57 hr⁻¹. The liquid medium contains the same constituents as in Example 1. Agitation in the reactor is provided by liquid recirculation, using a recirculation rate of 60 gpm. The operating pH in the reactor is 5.05. Under these conditions, the CO conversion is 57 percent and the H₂ conversion is 58 percent. A hollow fiber unit is used to maintain a cell concentration of 13.6 g/L inside the reactor.

The dilute acetic acid/acetate product stream containing 6.4 g/L combined acetic acid/acetate and 2 g/L acetic acid is sent to a three stage countercurrent extraction column. The solvent to feed ratio is 1:4. The acetic acid in the solvent leaving the extractor is 10 g/L. Water (medium) from the extraction unit is sent back as recycle to the reactor.

The solvent containing the acetic acid is sent to distillation to recover the acid and solvent. A vacuum solvent distillation column and an acetic acid distillation column are used in the separation. Glacial acetic acid is produced as the final product.

EXAMPLE 6

Production of Potassium Acetate from Carbon Black Waste Gas

The carbon black waste gas of Example 4 is used to make potassium acetate instead of CMA. All fermentation and solvent extraction conditions remain the same. Caustic potash (potassium oxide) is used to react with the acetic acid to form a 50 percent solution of potassium acetate directly in the solvent phase.

EXAMPLE 7

Production of SCP from Coke Oven Waste Gas

A coke oven waste gas containing about 6 percent CO, 2 percent CO₂, 57 percent H₂, 5 percent N₂, and 27 percent gaseous hydrocarbon is fed to a continuous stirred tank reactor with cell recycle as described previously in Example 4. The reactor is used to produce a product such as dilute acetic acid or ethanol. In addition, the cell concentration inside the reactor is 13.6 g/L. These cells (microorganisms) can be harvested to produce bacterial single cell protein as an animal feed. A purge stream from the reactor containing cells is sent to a dryer to process dry single cell protein.

EXAMPLE 8

Production of H₂ from Refinery Waste Gas

Refinery waste gas containing about 45 percent CO, 50 percent H² and 5 percent CH₄ is sparged into a 1 L CSTR operating at 50° C. and a few inches of water pressure containing Bacillus smithii isolate ERIH2 which was deposited on Mar. 18, 1993 with the American Type Culture Collection, Rockville, Md. and given deposit accession no. 55404. The medium to the reactor is 1.0 g/L corn steep liquor. CO in the waste gas is converted along with water to CO₂ and H₂. With a 90 percent conversion, the exit gas stream contains 3.2 percent CO, 64.4 percent H₂, 28.8 percent CO₂ and 3.6 percent CH₄. The CO, CO₂ and CH₄ are removed from the gas stream by solvent extraction.

EXAMPLE 9

Production of Other Chemicals from Carbon Black Waste Gas

Carbon black waste gas containing about 14 percent CO, 17 percent H₂ and 4 percent CH₄ in N₂ is sparged into a 1 L CSTR operating at 37° C. and a few inches of water pressure. The medium in the reactor is a basal salts mixture containing water, B-vitamins, salts and minerals. The single or mixed culture in the reactor produces a liquid phase product of methanol, propanol, butanol, propionic acid, butyric acid or other desirable product. The system is set up essentially the same as in Example 6. Following dilute product formation, the product is recovered in a suitable product recovery system consisting of extraction, distillation or other well-known product recovery techniques. If multiple products are produced, an stagewise product recovery system is employed.

EXAMPLE 10

Production of Ethanol from Oil Refinery Waste Gas

A mixture of oil refinery waste gases containing about 45 percent CO, 50 percent H₂ and 5 percent CH, is sparged into a 1.0 L continuous stirred tank reactor without cell recycle containing Clostridium ljungdahlii isolate O-52. This bacteria was deposited on Jul. 27, 1997 with the American Type Culture Collection (ATCC), Rockville, Md. under Accession Number 55989. The refinery waste gases are produced, for example, by mixing streams from the catalyst regenerator and the catalytic cracker, two unit operations commonly found in the refinery. The operating pressure inside the reactor is a few inches of water, and the temperature is 37° C. The gas retention time is 12 min and the liquid dilution rate is 0.042 hr¹. The liquid medium contains basal salts and vitamins with a nutrient limitation to enhance ethanol production. The Operating pH in the reactor is 5.05, and the agitation rate is 1000 rpm. Under these conditions, the CO conversion is 85 percent and the H₂ conversion is 50 percent. The ethanol concentration in the product stream from the reactor is 21 g/L. The acetic acid by-product concentration was 3 g/l.

The product stream from the fermenter is sent to distillation for product recovery. The bottoms from the distillation column containing acetic acid and water is sent back to the fermenter as water recycle. The overhead product from distillation containing 95 percent ethanol is sent to an adsorption unit to produce neat ethanol.

EXAMPLE 11

Production of Products from Waste Gas Using a Mixed Culture

The oil refinery waste gases of Example 6 are sparged into a 1.0 L continuous stirred tank reactor without cell recycle containing a mixed culture of bacteria capable of producing ethanol as the final product. The mixed culture contains one or more anaerobic bacteria that are capable of producing ethanol at low pH and under nutrient limitation. Other strains can also be present. The conditions inside the reactor are essentially identical to the conditions of Example 6. The product from the reactor is 15-20 g/L ethanol and 3-6 g/L acetic acid. The product stream from the reactor is treated identically to the method described in Example 6.

EXAMPLE 12

Production of Ethanol from Waste Gas Using BRI Isolate C-01

The oil refinery waste gases of Example 6 are sparged into a 1.01 L continuous stirred tank reactor without cell recycle containing a culture of Clostridium ljungdahlii isolate C-01. This bacteria was deposited on Jul. 27, 1997 with the American Type Culture Collection (ATCC), Rockville, Md. under Accession Number 55988. This bacteria is is capable of producing ethanol as the final product. The conditions inside the reactor are essentially identical to the conditions of Example 6. The product from the reactor is 20 g/L ethanol and 3 g/L acetic acid. The product stream from the reactor is treated identically to the method described in Example 6.

EXAMPLE 13

Production of Ethanol from Waste Gas Using C. ljungdnhlii PETC

The oil refinery waste gases of Example 6 are sparged into a 1.0 L continuous stirred tank reactor without cell recycle containing a culture of C. Ljungdahlii PETC capable of producing ethanol as the final product. The conditions inside the reactor are essentially identical to the conditions of Example 6. The product from the reactor is 15 g/L ethanol and 6 g/L acetic acid. The product stream from the reactor is treated identically to the method described in Example 6.

Thus, it will be appreciated that as a result of the present invention, a highly effective improved process for converting waste gases to acids, including organic acids, e.g., acetic acid, alcohols, hydrogen, SCP or organic acid salts is provided by which the principle objective, among others, is completely fulfilled. It is contemplated and will be apparent to those skilled in the art from the preceeding description and accompanying drawings that modifications and/or changes may be made in the illustrated embodiments without departure from the present invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, and are not limiting, and that the true spirit and scope of the present invention be determined by reference to the appended claims. 

What is claimed:
 1. A process for producing ethanol comprising the steps of:fermenting a gas composition comprising carbon monoxide anaerobically with a bacterial strain selected from the group consisting of Clostridium ljungdahlii strain O-52 ATCC No. 55989 and Clostridium ljungdahlii strain C-01 ATCC No. 55988 in an aqueous nutrient media in a bioreactor to produce a broth; removing continuously a portion of said broth from said bioreactor; and recovering the ethanol therefrom.
 2. The process according to claim 1, wherein the gas composition further comprises hydrogen.
 3. The process according to claim 1, wherein said gas composition is generated by an industrial process.
 4. The process according to claim 3 wherein the industrial process is selected from the group consisting of the manufacture of carbon black, the manufacture of ammonia, the manufacture of methanol, the manufacture of coke and petroleum refining.
 5. The process according to claim 1 wherein said bioreactor is selected from the group consisting of a continuously stirred tank reactor, an immobilized microbial cell bioreactor, a trickle bed bioreactor, a bubble column bioreactor and a gas lift bioreactor.
 6. The process according to claim 1 further comprising maintaining said bioreactor at a pressure of greater than one atmosphere.
 7. The process according to claim 1 wherein said recovering step comprises separating said ethanol and said bacteria by passing said removed broth containing ethanol through a cell separation unit, returning the separated bacteria to the bioreactor to maintain a high bacterial concentration and producing a bacteria-free, ethanol-containing stream.
 8. The process according to claim 7 wherein said separating comprises at least one of the steps selected from the group consisting of centrifugation, hollow fiber membrane filtration, settling and ultracentrifugation.
 9. The process according to claim 1 which is conducted in the absence of any cell separation.
 10. The process according to claim 1 wherein said recovering of said ethanol comprises the steps of (a) contacting said removed broth containing ethanol with a water-immiscible solvent having a high affinity for ethanol in a counterflow mixing vessel and then (b) optionally distilling the product of (a) to recover solvent and ethanol.
 11. The process according to claim 1 further comprising fermenting an additional bacterial strain selected from the group consisting of C. ljungdahlii ERI-2, Acetobacterium kivui, A. woodii, Butyribacterium methylotrophicum, Clostridium aceticum, C. acetobutylicum, C. formoaceticum, C. kluyveri, C. thermoaceticum, C. thermocellum, C. thermohydrosulfuricum, C. thermosaccharolyticum, Eubacterium limosum, Clostridium ljungdahlii PETC, Peptostreptococcus productus, Rhodospirillum rubrum and Rhodopseudomonas gelatinosa.
 12. The process according to claim 1 wherein said pH is between 4.0 and 5.5.
 13. The process according to claim 1 wherein said recovering of said ethanol comprises the step of distillation.
 14. A process for producing ethanol comprising the steps of:fermenting a gas composition comprising carbon monoxide carbon dioxide and hydrogen anaerobically with a bacterial strain selected from the group consisting of Clostridium ljungdahlii strain O-52 ATCC No. 55989 and Clostridium ljungdahlii strain C-01 ATCC No. 55988 in an aqueous nutrient media in a bioreactor to produce a broth; removing continuously a portion of said broth from said bioreactor; and recovering the ethanol therefrom.
 15. The process according to claim 14 wherein said gas composition is generated by an industrial process.
 16. The process according to claim 14 wherein the industrial process is selected from the group consisting of the manufacture of carbon black, the manufacture of ammonia, the manufacture of methanol, the manufacture of coke and petroleum refining.
 17. The process according to claim 14 wherein said bioreactor is selected from the group consisting of a continuously stirred tank reactor, an immobilized microbial cell bioreactor, a trickle bed bioreactor, a bubble column bioreactor and a gas lift bioreactor.
 18. The process according to claim 14 further comprising maintaining said bioreactor at a pressure of greater than one atmosphere.
 19. The process according to claim 14 wherein said recovering step comprises separating said ethanol and said bacteria by passing said removed broth containing ethanol through a cell separation unit, returning the separated bacteria to the bioreactor to maintain a high bacterial concentration and producing a bacteria-free, ethanol-containing stream.
 20. The process according to claim 19 wherein said separating comprises at least one of the steps selected from the group consisting of centrifugation, hollow fiber membrane filtration, settling and ultracentrifugation.
 21. The process according to claim 14 which is conducted in the absence of any cell separation.
 22. The process according to claim 14 wherein said recovering of said ethanol comprises the steps of (a) contacting said removed broth containing ethanol with a water-immiscible solvent having a high affinity for ethanol in a counterflow mixing vessel and then (b) optionally distilling the product of (a) to recover solvent and ethanol.
 23. The process according to claim 14 further comprising fermenting an additional bacterial strain selected from the group consisting of C. ljungdahlii ERI-2, Acetobacterium kivui, A. woodii, Butyribacterium methylotrophicum, Clostridium aceticum, C acetobutylicum, C. formoaceticum, C. kluyveri, C. thermoaceticum, C. thermocellum, C. thermohydrosulfuricum, C. thermosaccharolyticum, Eubacterium limosum, Clostridium ljungdahlii PETC, Peptostreptococcus productus, Rhodospirillum rubrum and Rhodopseudomonas gelatinosa.
 24. The process according to claim 14 wherein said pH is between 4.0 and 5.5.
 25. The process according to claim 14 wherein said recovering of said ethanol comprises the step of distillation.
 26. A biologically pure culture of Clostridium ljungdahlii strain O-52, ATCC Accession No.
 55989. 27. A biologically pure culture of Clostridium ljungdahlii strain C-01, ATCC Accession No.
 55988. 